It is known that a tin oxide thin film (SnO.sub.2 thin film) is transparent, conductive and chemically stabler than indium tin oxide film. And it is also known that the tin oxide thin film excels in light transmissive efficiency in the visible light region. In view of this, it is currently used as constituent members in various optical or/and electric devices.
Various methods for the preparation of such tin oxide thin film by means of spraying, thermal induced chemical vapor deposition, reactive sputtering etc. have been proposed.
Among those methods, the method of thermal induced chemical vapor deposition (hereinafter referred to as "CVD method") using tin tetrachloride (SnCl.sub.4) as a raw material is widely used in various applications. Under this method, the preparation of a tin oxide thin film is conducted in accordance with pyrolysis oxidation reaction or hydrolysis reaction which can be expressed by the following reaction formulas:
Pyrolysis oxidation reaction:
SnCl.sub.4 +O.sub.2 .fwdarw.SnO.sub.2 +2Cl.sub.2 PA1 SnCl.sub.4 +2H.sub.2 O.fwdarw.SnO.sub.2 +HCl.
and
hydrolysis reaction:
And the known CVD method for the preparation of a tin oxide film is generally implemented using such apparatus as schematically shown in FIG. 7, in which are shown carrier gas feeding pipe 1, flow meter 2, stop valves 3, 3', raw material vessel provided with an appropriate heating means (not shown), pressure gage 5, flow controlling valve 6, raw material gas supplying pipe 7, oxidizing gas supplying pipe 8, reaction vacuum chamber 9, substrate holder 10 having electric heater 10', substrate 11 and exhaust pipe 12.
The operation of forming a SnO.sub.2 thin film in the apparatus of FIG. 7 is carried out in the following way. That is, since Sncl.sub.4 as a film forming raw material is in liquid state at room temperature, carrier gas such as He gas is introduced into the SnCl.sub.4 liquid in the raw material vessel 4 so that the SnCl.sub.4 liquid being kept at a predetermined temperature may be bubbled with He gas (carrier gas) to provide a gas containing SnCl.sub.4. The resultant SnCl.sub.4 containing gas is successively introduced through the raw material gas supplying pipe 7 into the reaction vacuum chamber 9. In parallel, oxidizing gas such as O.sub.2 or H.sub.2 O is introduced through the oxidizing gas supplying pipe 8 into the reaction vacuum chamber 9. The thus introduced SnCl.sub.4 containing gas and oxidizing gas are engaged in the above mentioned pyrolysis oxidation reaction or hydrolysis reaction to thereby result in forming a tin oxide film on the substrate 11 being kept at a predetermined temperature by the action of the electric heater 10'. However, the known method is problematical due to the fact that it is practiced under elevated temperature conditions and other problems are associated not only with the elevated temperature conditions but also with the apparatus to be used. That is, because the known method is practiced while maintaining a substrate at an elevated temperature, firstly the kind of a substrate to be used is limited to one that has a sufficient heat resistance and that does not contain a material which can migrate and case changes in the characteristics of a layer to be formed on the substrate. Then, should such selected substrate be used, the layer to be formed is apt to be damaged with the action of heat, undesired side-reactions are apt to be caused between the substrate and a layer to be formed, whereby changes are brought about in the characteristics and the thickness of a layer to be formed is likely to be varied, whereby the resulting layer lacks uniformity of the thickness and homogeneity of the composition, which may itself also cause changes in the characteristics.
In this regard, even if a desirable tin oxide thin film should be fortunately obtained, the thin film will become costly.
Against this background, there is now an increased demand for providing a method that makes it possible to practice the process at low temperature and at a high film forming rate in a simple apparatus to mass-produce a desirable tin oxide thin film which has good uniformity and has practically applicable characteristics.